This invention relates to the molten metal level control in vertical, continuous or semicontinuous casting assemblies and particularly to such casting units wherein an electromagnetic field generated by an annular inductor surrounding the column of molten metal is used to shape the solidifying metal.
Aluminum ingots or billets which have been continuously or semicontinuously direct chill (DC) cast in conventional, open ended, tubular molds are usually characterized by various degrees of surface defects such as cold folds, liquations, hot tears and the like, which result primarily from the contact between the mold and the solidifying embryonic metal shell. Conventional DC cast ingots or billets are also characterized by considerable alloy segregation at the surface due to the sequential steps of initial cooling and partial solidification of the molten metal surface from contacting the chill surfaces of the mold bore, the reheating of the metal surface after the metal contracts away from the mold bore and then the final solidification of the molten metal by conduction toward the direct application of coolant. The conventionally DC cast ingots or billets usually require scalping to remove both the surface defects and the alloy impoverished zone adjacent to the surface before subsequent fabrication such as rolling, forging and the like.
Electromagnetic casting is quite similar to conventional DC casting except that, instead of the tubular shaped mold used in the conventional process, an annular inductor is employed to generate an electromagnetic field around the column of molten metal which in turn develops radial pressure on the column of molten metal sufficient to control the shape thereof until the metal is solidified into its final shape by the direct application of coolant. In the operation of electromagnetic casting units, no contact is made with the embryonic metal shell during solidification, so the aforementioned surface defects are for the most part eliminated. Moreover, due to the lack of contact between the embryonic metal shell and a chill surface of a mold bore, there is essentially no alloy impoverished zone adjacent to the ingot or billet surface and thus the electromagnetically cast metal is very homogeneous throughout its entire cross section. There is usually no need to scalp the electromagnetically cast material prior to fabrication and, as an added feature, the homogeneous structure reduces considerably or eliminates the edge cracking characteristic of conventional DC cast ingot during hot rolling.
The electromagnetic field in EM casting is produced by a ring-type inductor and the interaction of the electromagnetic field generated by the inductor with the eddy currents induced in the molten metal within the inner peripheral area of the inductor generate the electromagnetic pressure which controls the cross-sectional shape of the solidifying metal. The radial force components control the lateral position of molten metal and nothing contacts the solidifying molten metal until coolant is applied to the metal surface as it emerges from the bottom of the inductor. Solidification of the metal is effected primarily by the axial conduction of heat away from the molten metal toward the portion of solidified metal on which the coolant is applied.
The inductor is preferably powered by a high frequency electrical source (e.g. 500 to 15000 cycles per second) because at the higher frequencies the induced currents in the molten metal concentrate at the surface of the solidifying metal (commonly termed "skin effect") so there is very little turbulence caused in the body of the molten metal.
Further information on the principles of electromagnetic casting can be found in U.S. Pat. No. 2,686,864 (Wroughton et al) USSR Inventor's Certificate No. 233186 and U.S. Pat. Nos. 3,467,166; 3,605,865; 3,646,988; 3,702,155; 3,773,101; 3,985,179 and 4,004,631.
In order for ingot or billet to be EM cast with constant cross-sectional dimensions along the axial length thereof, the radial component of the electromagnetic pressure must continually be in a dynamic equilibrium with the hydrostatic pressure of the molten metal. Exercising the control necessary for the dynamic equilibrium is considerably more difficult than it first appears because minor changes in the electromagnetic field, in the drop rate, or in the height of molten metal can have a significant effect on the cross-sectional dimensions of the resultant ingot or billet. Care must be exercised, particularly during startup, because a localized pressure of molten metal can exceed the radial electromagnetic pressure resulting in excursions of molten metal over the bottom block which, when solidified, resemble icicles on the butt end of the ingot or billet. While it is difficult enough to cast one ingot or billet having constant cross-sectional dimensions along its length, it becomes even more difficult to exercise such control when a plurality of ingots or billets are cast at the same casting station.
Several schemes have been developed in an attempt to overcome the dimensional control problems but none appear to have been widely accepted. In U.S. Pat. No. 4,014,379 the electrical current level to the inductor is controlled in response to deviations sensed in the height of the molten metal in the inductor. Similarly, although apparently limited to copper, in U.S. Pat. No. 4,161,206 the electrical current to the inductor is controlled in response to deviations in the distance between the inner surface of the inductor and the vertical surface of the column of molten metal. In both cases the electrical current level in the inductor is varied to adjust the electromagnetic pressure to compensate for any differences between the distance measured and that desired.
Although these prior processes may control the dimensions of the ingot or billet to a certain extent, the dimensional control is believed considerably less accurate than that desired. Moreover, these processes by themselves are not readily amenable to controlling the electromagnetic casting of a plurality of ingots or billets in a single casting station. It is to these problems that the present invention is directed.